Optimum applications of nitrogen fertilizer and water regime for wheat (Triticum aestivum L.) using 15N tracer technique under Mediterranean Environment

Two field experiments were conducted under two different soil texture located in two sites. Tracing the nitrogen status in wheat plants – soil - environment system using 15N technique under different water regimes was the aim of this investigation. Three nitrogen rates (100, 80 and 60% of the recommended rate with 5% 15N atom excess labeled ammonium sulfate) through two mode of application (Mode A; nitrogen fertilizer rate splitting into three doses: 25, 25, 50% of added rates at seedling, tillering and jointing, respectively, and Mode B; splitting N rate into two doses: 35 % at seedling and 65% at tillering) conjugated with three water regimes (100, 75 and 50% of crop water requirement (CWR)) were applied under clay loam and loamy sand soils conditions. Wheat grain and straw yields were declined by 16.4 and 4.4% in clay loam soil, and by 34.6 and 20.7% in loamy sand soil as water regime reduced from 100 to 50% CWR, respectively. Application of 80% of the recommended N rate recorded the highest grain yield under Mode A, whereas 100% N rate recorded the highest straw yield under Mode B in both soil textures. The moderate rate of N fertilizer (80%, corresponding to 142.4 and 190.4 kg N ha-1 in clay loam and loamy sand soil, respectively) applied with mode A under 75% water regime (4017.7 and 4200.8 m3 applied irrigation water per hectare in clay loam and loamy sand soil, respectively) recorded the highest N uptake by plant, nitrogen use efficiency (NUE) and consequently wheat yield.